Observation of Multiple Stop Bands in Photonic
Bandgap Structures Doped with Organic Dyes**
By Petr Nozar,* Davide DiDomenico, Chiara Dionigi, Maria Losurdo, Michele Muccini, and Carlo Taliani
The dramatic growth of the number of papers dealing with photonic bandgap (PBG) structures indicates the importante of the subject.[1-151
In this paper we report on the optical properties of self-as¬sembled periodic structures of monodisperse doped polysty¬rene beads (LATEX).[16-21 Our aim is to study the enhance¬ment of the refractive index contrast using an organic dye (Oil Red EGN) infiltrated into the periodic structure and the influence of the dye absorption on the optical properties of the assembly. We test the validity of the simple analysis of op¬tical spectra in the framework of the Bragg and Snell law. Some considerations regarding dimensions of LATEX suit¬able for experiments are also drawn.
Similar experiments have been performed by Park et al. on LATEX doped with the organic dye Oil Blue N aggregated as a thin layer ilin).[20] Measurements of optical spectra re¬ported in the literature[17231 were performed in transmission configuration; they suffer from some basic disadvantages: Firstly, the wavelengths of the stop band of doped and un¬doped samples are generally different; secondly, due to the multiple scattering in photonic crystals (PCs) the effective op¬tical thickness of doped and undoped samples are different even if the physical thickness is the same; and thirdly, the ab¬sorption in the doped PC is strongly amplified due to multiple scattering, in contrast to the disordered doped sample. There¬fore, it seems to be inadeguate to use both ordered undoped PCs and disordered doped samples as references for the ab¬sorption in the ordered doped PC. Indeed, from our transmis¬sion measurements (not reported here) it follows that even at the lowest concentration of dye (1.3 x 10-3 M) used, the ab¬sorption of the dye reaches saturation, which manifests itself as an apparent bandgap in the transmission spectrum, inde¬pendent of the incident angle of the light beam.